In commonly owned application Ser. No. 860,835, filed Dec. 15, 1977 by Guiliano Brambilla, there has been disclosed and claimed a filter for a telecommunication system, designed particularly for frequencies in the microwave range, wherein such a coil is disposed inside a resonant cavity and has an intermediate tap by which one of its turns is connected to an input lead. One of the two ends of the coil is left unconnected while its opposite end is grounded at an adjoining cavity wall. The coil body is wound on a hollow core of low-loss dielectric material whose interior is partly occupied by a metal rod designed as a screw which may be axially shifted to vary a distributed capacitance existing between that rod and the portion of the coil lying between the free end and its tap. A lumped capacitor, also adjustable, is connected between the input lead and ground.
If the coil is wound from an ordinary round wire, the joining of the input lead to one of its turns by the usual soft-soldering method (using tin) creates difficulties since the heating of the wire may affect other connections within the cavity, such as the solder joint securing the grounded end of the coil to the cavity wall. Recourse is therefore frequently had to electric spot or seam welding which, however, cannot be easily performed with the required degree of precission as concerns the location of the tap along the coil; as is known, even a small change in that location may materially alter the electrical characteristics of the filter. Furthermore, the grounding of a coil terminal through a solder joint introduces a significant series resistance tending to lower the Q-value of the circuit.